Metal oxides, in particular mixed metal oxides have a broad range of applications such as e.g. ceramics, polymer additives, fillers, pigments, reactive surfaces, catalysts, storage materials, polishing additives, membranes, fuel cells etc. Among the most important metal oxides are cerium oxides and in particular cerium-zirconium mixed oxides, below referred to as ceria/zirconia. Ceria/zirconia are used in the current new generation of Three Way Catalysts (TWC) as key-component due to their high dynamic oxygen exchange capacity [Trovarelli et al. (1996), Kaspar et al. (1999)], however also as Oxidation Catalysts, Ceramics, Polishing agents and Fuel cells, amongst others.
In the treatment of noxious gases from the car exhaust, the ceria switches between its two major oxidation states Ce(III) and Ce(IV) thereby taking up or releasing electrons for the conversion of CO and residuals from the combustion process. Depending on the oxygen partial pressure, it absorbs or releases oxygen and stabilizes the air-to-fuel ratio making CO oxidation a fast and reliable process [Taylor (1984)]. It is well established, that the addition of zirconia to ceria as a solid solution greatly enhances the reducibility of the Ce(IV) [Kundakovic (1998); Balducci (1995)]. Different production methods, however, lead to a varying state of molecular mixing of ceria and zirconia. Maximum stability is found for intensively mixed powders forming a stable solid solution of zirconia in ceria. The such formed stable phase can contain more than 30 atom-% zirconium. However, most preparation method result materials unstable at higher zirconia content. The mixed oxides then forms two or more phases of different composition. This may lead to reduced overall temperature stability.
Current methods for the production of metal oxides such as ceria and ceria/zirconia are mechanical and mechanical/thermal processes, wet-phase chemistry based methods, and high temperature methods such as flame spray pyrolysis (FSP).
Mechanical and mechanical/thermal methods are energy intensive (milling!) and generally suffer from insufficient mixing at the atomic level leading to low phase stability and/or low specific surface area:
Wet-phase based methods entail huge solvent costs, produce large amounts of waste water and need calcination steps after the synthesis, making them cost intensiv. Furthermore, although e.g co-precipitation of ceria/zirconia can lead to mixed oxide powders with extremely high specific surface areas, unfortunately, the temperature stability of as-prepared oxides is characterized by a big loss of specific surface area at elevated temperature. Preparation at high temperature may produce an oxide with increased stability. This has prompted several people to attempt to prepare ceria by spray pyrolysis. Flame spray pyrolysis (FSP) is a known process and has been used for preparation of many oxides. However, in the case of ceria and in particular ceria/zirconia, the research for suitable precursors entails huge problems associated with the chemical properties of cerium and zirconium compounds. For example Yoshioka et al. (1992) used FSP for the production of ceria oxides, but they received a powder of low specific surface area. WO 01/36332 discloses a FSP method leading to an inhomogeneous product comprising ceria particles of broadly varying sizes. Aruna et al. (1998) investigated the ceria/zirconia synthesis by combusting mixtures of redox compounds and oxidizing metal precursors. This high temperature preparation yielded a high surface area product with apparently good phase mixing in as-prepared powders. However, the preparation of ceria/zirconia by solid combustion is difficult to realize at high production rates, since the process may quickly run out of control. Furthermore it is basically a batch process and the reproducibility is a general problem. Laine et al. (1999) and Laine et al. (2000) used a spray pyrolysis unit to prepare ceria/zirconia but the specific surface area of the product powder stayed low, at 10 to 15 m2/g. EP 1 142 830 also discloses a FSP method for the preparation of ceria/zirconia starting from organometallic compounds in organic solvents and/or water. The procedure disclosed in EP 1 142 830 focuses on chlorine free powders produced by flame spray pyrolysis and uses precursor solutions of type MeR where R is an organic rest such as methyl, ethyl, or a corresponding alkoxy group or a nitrate anion. As solvents, water or alcohols are used.
Recently Mädler et al. (2002B) disclosed an FSP method for the production of pure ceria with high surface and homogeneous particle sizes. This solvent system, however, has now been found to be unsuitable for the production of e.g. ceria/zirconia.
Therefore there is still a need for a high temperature method for the production of metal oxides, in particular mixed metal oxides that leads to a product with increased homogeneity of the product.